Watch episodes Jolly Holly Secrets I & II (both of them, it's a two parter, but the big revelation is in II).

Basically it turns out the Ice King (who up until this point was basically just a silly villain who kidnaps princesses) was a human before the Great Mushroom War (a nuclear war between humans that made humans extinct and mutated several creatures into the current land of Ooo) named Simon Petrikov, who bought a crown that slowly made him lose his sanity every time he wore it, eventually he went insane.

The episode "I remember you" also shows that Marceline and Simon were good friends, before Simon completely lost himself, as right after the war Marcy was just a little girl and thus Simon had to protect her. We also learn that the crown is the only thing keeping Simon alive at this point.

In the episode "Simon & Marcy" we basically see Simon and Marcy slightly after the war trying to survive on a post nuclear earth before it became the land of Ooo. Here we find out how Simon eventually went completely insane as he constantly had to put on the crown to protect Marcy from the mutated creatures. It doesn't get to the point where he completely loses himself in the episode but we know he did later down the road as 1000 years later (where the show currently takes place) he no longer remembers anything of his life as Simon.

It's a show about (who we later learn to be mutated) candy people, and 1000 year old insane and occasionally homicidal pervert, an insane and often homicidal lemon man with vocal cords of steel, a 13 year old homicidal maniac who punches everything with a face, ext... Pointing out the impossibilities is a little moot.

I just realized there are only a handful of characters in the entire show who aren't insane and homicidal.